Composite tethers (also referred to as cables, tendons, support lines, mooring lines and the like) are useful for securing floating structures such as TLPs in deepwater. Particularly in depths over about 4000 feet, composite tethers offer significant economic and technical advantages and reliability over steel tethers. Composites such as carbon fibers embedded in a polymer matrix material are lightweight and have high specific strength and stiffness and excellent corrosion and fatigue resistance, which make them attractive for water depth sensitive components such as tethers and risers or umbilicals, which transport hydrocarbons from a wellhead on the ocean floor. Furthermore, composites are easily outfitted with instrumentation such as fiber optics integrated into the composite for load and integrity monitoring.
Conventional composite TLP tethers comprise top and bottom end connectors for connection to the TLP and a foundation on the ocean floor, respectively, and a tether body having a plurality of parallel twisted strands. The twisted strands herein referred to are formed from a twisted assemblage of small, parallel rods having a diameter of about 3-6 mm, and typically comprise in the range of about 50 to 200 rods per strand, wherein the assemblage of rods is subjected to a helical twist, typically about 2 to 3° on the outer rods. The plurality of parallel twisted strands, wherein each strand is typically about 50 to 75 mm in diameter, are also twisted slightly to achieve a helix in the conventional tether, also referred to herein as a twisted tether. The size of the conventional tether is determined by the number of twisted strands, which is dictated by the strength and axial stiffness requirements for a given tether service (e.g., size of the TLP, water depth, ocean currents, storm history, etc.). The number of twisted strands per conventional tether is typically from about 8 to 30 twisted strands per assembled conventional tether. Conventional tethers are twisted as described previously so that they may be wound upon tether spools, typically having a diameter of greater than about 4.0 meters, and preferably from about 4 to 8 meters. In order for the conventional composite tethers to be spoolable, small diameter rods having a diameter of no greater than about 6 mm are required, otherwise the size of the required spool becomes impractical, as described below. The spooled tethers are transported upon reel ships or barges for installation and anchoring of the TLP to the ocean floor.
The manufacturing process of a conventional, spoolable composite tether includes the following steps: fabrication of small diameter composite rods, assembly of the rods into twisted strands, assembly of the twisted tether from multiple twisted strands (including addition of filler and profile material as needed), and termination of the twisted strands in top and bottom end connectors of the tether. The manufacturing of conventional, spoolable composite tethers is described in the following conference paper, which is incorporated by reference herein in its entirety: Composite Carbon Fiber Tether for Deepwater TLP Applications, presented at the Deep Offshore Technology Conference held in Stavanger, Norway on Oct. 19-21, 1999.
Composite materials for rod manufacture consist of small diameter fibers (from about 6 to about 10 microns) of high strength and modulus, preferably carbon fibers, embedded in a polymer matrix material, e.g., resins or glues. Commonly known thermoset or thermoplastic polymeric matrices may be used. Preferred matrix materials include vinylesters and epoxies. The resin materials have bonded interfaces which capture the desirable characteristics of both the carbon fibers and the matrix. The carbon fibers carry the main load in the composite material while the matrix maintains the fibers in the preferred orientation. The matrix also acts to transfer load into the carbon fibers and protects the fibers from the surrounding environment. Carbon fibers incorporated in the matrix may be spun in long continuous lengths; however, short (from about 25 to about 100 mm) discontinuous fibers may also be used.
Composite rods are typically manufactured by pultruding the composite material comprising the carbon fibers and the polymer matrix material. Pultrusion is the pulling of the resin wetted fibers through a die rather than pushing it through the die as in extrusion processes used for metal manufacturing. The die size and shape control the final size and shape of the pultruded composite product. There are several commercial pultruders such as Glasforms, Inc., DFI Pultruded Composites Inc., Exel Oyj, Strongwell Corp., Spencer Composites Corp., and others that are capable of producing the composite rods. Rods used in conventional spoolable tethers are typically round in cross-section. The composite rods produced typically have a weight which is approximately ⅙ that of required for an equivalent steel rod. As discussed previously, rods for use in conventional composite tethers typically are from about 3 to about 6 mm in diameter and are often wound onto rod spools, for example a 1.8 or 2.2 m diameter rod spool, for transportation to a strand and/or tether manufacturing facility.
In general, it is desirable to increase the stiffness of rods used in a tether, and the stiffness of a rod may be calculated according to the following equation:
      E    ·    A    =            4      ·              π        2            ·      L      ·              (                              Vertical            ⁢                                                  ⁢            Mass                    +                      Added            ⁢                                                  ⁢            Mass                          )                    n      ·              T        2            where E=axial stiffness of a rod (Pa); A=cross sectional area of 1 tether (m2); L=water depth (m); n=number of tethers; T=heave natural periods (s), typically from about 5 to about 5.5 seconds; vertical mass=mass of the platform (kg); and added mass=mass of the water that moves when the platform moves (kg). Typically, a stiffer rod cannot be bent as much as a less stiff rod. Given that the rods typically must be wound onto a rod spool for transportation, the bending stiffness of the rod is proportional to the diameter of the rod (d) raised to the fourth power (i.e., d4). Thus, it is necessary to use a small diameter composite rod (i.e., from about 3 to about 6 mm) in order for the resulting rod spool diameter to be a practical size for handling and transport and the force necessary to spool the rod and maintain it on the spool be practical. More specifically, in sizing the rod spool, the strain in the spooled rod is equal to the diameter of the composite rod divided by the diameter of the rod spool. In a properly sized spool, the rod strain is less than 50% of the ultimate strain to failure of the rod. Thus, if the composite rod has 1% strain to failure, then the diameter of the rod spool then needs to be larger than 200 times the diameter of the rod to be able to spool the rod onto the rod spool without damaging the rod. If the composite rod has ½% strain to failure, then the diameter of the rod spool has to be larger than 400 times the diameter of the rod. The diameter of a spool refers to the hub or core (i.e., drum) of the spool. In sum, where the rod itself must be spooled (or a strand or tether incorporating the rod must be spooled, as discussed below), the diameter and/or the stiffness of the rod must be engineered accordingly.
In a conventional, spoolable composite tether, the rods are assembled into bundles to form twisted strands. The twisted strands can be manufactured using typical wire rope stranding methods. Specifically, the rods are uncoiled from the rod spools and pulled through a guide plate for bundling. When the required number of rods per strand are laid out, the guide plates are rotated to impart a slight helical twist, typically 2 to 3° on the outer rods. Twisting the strand provides sufficient coherence to the strand for handling, coiling and transportation without significantly affecting the axial strength and stiffness. The rods in the twisted strands are fixed into a position by wrapping with tape or other securing device, cut to length and spooled onto strand spools for use in the assembly of the tether body. Generally, twisted strand spools include 1.8 or 2.2 m diameter spools such as those used for rod spools.
The twisted strands are assembled to form a conventional, spoolable composite tether [5] (i.e., a twisted tether) as shown in FIG. 1. The conventional, spoolable tether [5] is made up of multiple twisted strands [15], the twisted strands being further twisted with each other to form the twisted tether [5]. It can be seen that there are a large number of composite rods [10], which are bundled together to form individual twisted strands [15]. In this particular figure, there are fifteen twisted strands [15] making up the twisted tether [5], and a typical conventional tether may include from about 8 to about 30 twisted strands. The twisted strands [15] are held in place by a profiled member [20], which fills the voids between the twisted strands [15] and also provides a means for imparting a helical twist to the plurality of twisted strands [15] (thereby forming the twisted tether [5]). The profiled member [20] is preferably made from a plastic such as polyvinylchloride (PVC) or polypropylene and may be divided into segments such as center profile [25], intermediate profile [30], and outer profile [35]. The profiled member [20] may also contain void spaces [40]. A filler material may be placed in the void space between the twisted strands [15]. Preferred filler according to this invention is foam, which is used to give the tether buoyancy as described below.
The twisted strands [15] are free to move individually in the length direction, allowing individual adjustment and hence a better distribution of axial loads. The composite rods [10] and twisted strands [15] are free to act or move independently in the twisted tether [5]. In other words, there is relative axial movement between adjacent composite rods [10] within a twisted strand [15] and between adjacent composite twisted strands [15] within a twisted tether [5]. Otherwise the entire diameter of the conventional, spoolable tether [5] must be considered in calculating the diameter of the tether spool, since strain relates to the diameter of the body that is being spooled divided by the diameter of the spool, as described previously. By putting a twist in the composite rods [10] (via twisted strands [15] and twisted tether [5]) and keeping them separate and independent, the diameter of the individual composite rods [10] can roughly be used to calculate the diameter of the tether spool rather than the entire diameter of the twisted tether [5]. Typically, however, the tether spool is made somewhat larger to account for the friction between adjacent composite rods [10] as the conventional tether is spooled onto the tether spool.
As shown in FIG. 2, different size twisted strands [16] and [17] may be used to better fit all the twisted strands [16] and [17] inside an outer jacket or casing [45]. Preferably, the area within the casing [45] is filled by twisted strands [16] and [17] and empty void spaces are minimized. The twisted strands [16] and [17] are typically at least 30% of the area of the conventional, spoolable tether [5] and more typically 50% of the area of the tether [5]. Typically, the profiled member [20] and any filler material do not add any performance characteristics to the twisted tether [5], and thus it is desirable to minimize such components as much as possible to avoid unwanted additional weight and increased size.
The assembly of the conventional, spoolable tether [5] is performed using a conventional umbilical closing machine. Spools containing the twisted strands [15] and the profiled members [20] are lifted onto the closing machine. The twisted strands [15] and the profiled members [20] are then pulled through closing plates. During this process, the machine rotates to impart a helical twist in the twisted strands [15] to form the twisted tether [5]. A yarn or other securing device is then applied to hold the assembly together prior to extrusion of the protective, outer jacket [45] such as high density polyethylene (HDPE), nylon, or the like over the twisted strands to hold the twisted strands in place and protect the tether during handling. Conventional, spoolable tethers may be manufactured as a single continuous body that is spooled onto a spool. Alternatively, the tether body may be manufactured as a plurality of body lengths or segments that are spooled onto a spool. The segments are connected with connectors (e.g., couples or collars) to create a continuous tether. Segmenting the tether is helpful in accommodating production of rods, strands, and tethers, in limiting spool size, and in readjusting tether length for re-installations.
The final step of manufacturing a conventional, spoolable composite tether is the termination process that includes connecting the twisted tether to top and bottom end connectors. Termination using resin-potted cones has been extensively used in the wire rope industry. Resin terminations have been proven to be successful for terminating composite twisted strands as well. The twisted strands are fastened to a steel end connector using a potted cone technique similar to that used for termination of steel wires. The twisted strands are spread with a specific angle in the steel cones, and the cone is then filled with epoxy resin. A vacuum injection method is used in this process to avoid air gaps and to ensure consistent molding. Use of a flexible cone and cylindrical metal connector with spacers can minimize the effect of termination bending and provide better rod distribution inside the end connector. Alternatively, the twisted strands may be individually terminated and then assembled into a tether. After termination, the tether is spooled onto an appropriately sized conventional tether spool having a drum diameter of from about 4 to 8 m and a width of about 5 m, for transportation and installation offshore. An appropriately sized tether spool should be selected based upon the characteristics of the composite rods as described previously.
When conventional tethers are spooled, the inside rods comprising the twisted strands (and the inside twisted strands comprising the twisted tether) are spooled at a smaller diameter than the outside rods comprising the twisted strands (and the outside twisted strands comprising the twisted tether), thereby affecting the positioning of the rods (and twisted strands) within the conventional tether and the compression and strain forces acting thereon. Twisting the individual strands (i.e., twisted strands) and the conventional tether itself (i.e., twisted tether) subject the rods comprising the twisted strands and the twisted strands comprising the twisted tether to an effective “average diameter,” meaning that no individual rod or twisted strand is always on the inside or outside of the spool. Thus, all of the rods comprising the twisted strands and the twisted strands comprising the twisted tether maintain relative position to one another and experience about the same forces while on the spool.
A number of problems exist with conventional, spoolable composite tethers. Attempts to maximize the tether stiffness are limited by the requirement that the rod diameter and/or stiffness be engineered such that the rods (as well as the resultant twisted strands and twisted tether) may be spooled without damage to the rods. Spoolable tethers incorporating a large number of rods are more difficult to manufacture and handle, and result in larger diameter tethers that are more susceptible to adverse affects from wave action such as fatigue and possible failure over time. Rod strands typically result in more undesirable void space within the tether since the strands often cannot be tightly spaced, further requiring more filler material and/or profiled members that add undesirable weight and increase size. The required twist in the twisted strands and in the twisted tether to facilitate spooling also adds to the difficulty and cost of manufacture and reduces the axial stiffness of the spoolable tether, thus requiring a larger number of rods to compensate for the stiffness loss. Expensive reel ships are required for transport and installation of spoolable tethers on TLPs. The novel composite tether of the present invention solves these various problems.